Should I be given my retiring job share partner’s hours?

A few years ago I started a job sharing role. In April this year, my job share partner retired. My immediate line manager asked me if I was interested in taking up my partner’s hours. I said yes as it would give me a full-time Monday to Friday post. I currently have two contracts with the same employer.  Due to Covid restrictions, etc, I covered some of my job share partner’s hours by working overtime. When things began going back to normal, I returned to my workplace and contacted my line manager advising I was happy to take on the job share hours any time and resign from my other weekend role. My manager told me that due to restructuring in my department I wouldn’t be getting the job share hours as HR had told her it was being given to a person who was being re-deployed. I told her I wasn’t happy with this and wanted to speak to HR as my contract stated I was to be offered the job share hours. She told me re-deployment ‘trumped’ my contract. I spoke to the member of HR who said that back in February, after the budget was agreed, all vacancies were put on hold and now due to restructuring the job was being filled by re-deployment. I explained I was unhappy about this and told her about about my two contracts. If the person was being re-deployed, why could they not be given my weekend post? She advised she would look into this and get back to me. What are my rights in this situation?